Which cognitive abilities are important for school achievement? To what extent are relationships between cognitive abilities and school achievement mediated genetically? Do these relationships or their genetic and environmental etiologies differ across the range of ability? These critical questions for the study of mental retardation are the focus of the proposed research, the first behavioral genetic study to address these issues. Little is known about the etiology of relationships between tests of cognitive processes and measures of school achievement, especially at the lower end of the distribution of cognitive abilities. In the proposed 3-year research program, a battery of cognitive measures and school achievement data will be obtained for 150 pairs of identical twins and 150 pairs of same-sex fraternal twins from 7 to 12 years of age in order to assess the relative contribution of genetic and environmental influences on the convariance between cognitive abilities and school achievement. The twins will be sampled for IQ so that three normally distributed subsamples of maximum size and equal standard deviations can be assembled from the total sample. Subsamples will have means of 85, 100, or 115, and will be used to explore whether genetic and environmental etiologies of the ability/achievement relationship differs across the population range. Non-twin siblings in 100 twin families within the same age range will also be tested in order to assess the extent to which twins share more common family environment than do non-twin siblings. Three types of cognitive ability measures will be employed: 10 tests of elementary cognitive processes administered by computer, paper-and-pencil psychometric tests of group factors of specific cognitive abilities, and WISC-R IQ. School achievement will be assessed using standardized achievement test scores. The theoretical significance of this research lies in the description and explanation of relationships among elementary cognitive processes, psychometric measures of cognitive abilities, and school achievement. The practical significance lies in the diagnostic and possible therapeutic benefit for understanding mild mental retardation in the context of the general population.